Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging. Particular embodiments relate to three-dimensional multi-spectral imaging (3D MSI) for use in the presence of metal.
Discussion of Art
In magnetic resonance imaging (MRI), when human or other animal tissue is subjected to a uniform magnetic field, i.e., a polarizing field B0, the individual magnetic moments of particle spins in the tissue attempt to align with the polarizing field, but precess about the field in random order at their characteristic Larmor frequency. If the tissue is subjected to an RF magnetic field, i.e., excitation field B1, which defines an x-y plane and varies at a frequency near a Larmor frequency of selected particles, the net aligned moment, or “longitudinal magnetization” of those selected particles, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment. After B1 is terminated, the tipped spins “relax” back into the precession defined by B0, and, as a result, produce RF signals. The RF signals may be received and processed to form an image. In order to form a pixelated image for human interpretation, gradient magnetic fields, Gx, Gy, Gz, are applied to localize the tissue response to B1.
Paramagnetic material such as joint implants or bone screws, for example, can create regions of distortion in the polarizing field B0, which detract from accurate localization of the tissue response to excitation. As one solution, 3D multispectral MRI protocols have been demonstrated using proton-density (PD) and short-tau inversion recovery (STIR) contrasts, and have proven effective for removing bulk distortions around metal implants. It has been demonstrated that heavy frequency overlap of spectral components in 3D multispectral MRI can aid in reducing residual image artifacts. In particular, 3D MSI techniques such as SEMAC, MAVRIC, and MAVRIC SL can substantially reduce bulk-susceptibility artifacts that cofound conventional methods applied in the presence of metal. A common remaining artifact in 3D MSI techniques, however, is the ring artifact found in regions near implants with strong local magnetic induction field gradients. Ring artifacts are limited to the frequency-encoded dimension of a 3D MSI image. The rings follow the progression of the local magnetic induction field gradient as an implant interface is approached. In particular, the compass directions and magic angles relative to implant constructions are prone to the most severe gradients, due to the stronger derivative in these regions of the induced dipole-dominated field distributions.
In view of the above, it is desirable to provide apparatus and methods for repairing ring artifacts. Such apparatus and methods might also be helpful toward repair of oriented image artifacts, generally, e.g., eliminating lens flare in visible light photography.